The present invention generally relates to a communication cable for use in a plenum and, in particular, relates to one such communication cable having one or more first twisted pairs of electrical conductors having a first insulating material about each electrical conductor thereof and one or more second twisted pairs of electrical conductors having a second insulating material about each electrical conductor thereof wherein the first and second insulating materials are different.
As the demands for communication services have increased, it has become necessary to provide communication cables in larger and larger numbers. This is particularly true in office buildings where more and more communication services are being demanded. Typically, rather than rewire an entire existing building, it has been found more economical to provide the needed communication services by running the requisite communication cables in plenums. In general, a plenum is defined as a compartment or chamber to which one or more air ducts are connected and which forms part of the air distribution system of the structure. Generally, in existing buildings, communication cables are readily provided within the areas above drop ceilings in the portions of the facility being rewired. These plenums are, typically, return air plenums. Alternatively, plenums can also be created beneath a raised floor of a facility.
From the above it can be readily understood why it would be very advantageous to utilized a wiring scheme within these fairly accessible places. However, since these plenums handle environmental air, considerable concern regarding a fire incidence is addressed in the National Electrical Code by requiring that communication cables for use in plenums pass a stringent flame and smoke tests. Consequently, in the manufacture of communication cables the fire resistance ratings that allow for installation within certain areas of a building, particularly plenums, are of primary importance.
Currently, communication cables for use in plenums must meet the requirements of the Underwriter's Laboratory Standard 910 which is entitled Test Method For Fire and Smoke Characteristics of Cables Used In Air-Handling Spaces. This is a well known test performed in a modified Steiner Tunnel. During the test, a single layer of 24 foot lengths of cable are supported on a one foot wide cable rack that is filled with cables. The cables are ignited with a 300,000 Btu/hr methane flame located at one end of the furnace for a duration of 20 minutes. Flame spread within the tunnel is aided by a 240 ft/minute draft. Flame spread is then monitored through observation windows along the side of the tunnel. Concurrently, smoke emissions are monitored through the use of photocells installed within the exhaust duct. This is a severe test that to date has been passed only by communication cables using premium materials such as low smoke materials, for example, Fluroethylenepropylene (FEP), Ethylene-chlorotrifluoroethylene (ECTFE), or Polyvinylidene fluoride (PVDF). In general, communication cables passing this test are approximately three times more expensive than lower rated cables designed for the same communication application. However, communication cables falling this test must be installed within conduit, thereby eliminating the benefits of an economical, easily relocatable cable scheme.
In general, the manufacture of communication cables are well known, for example, U.S. Pat. No. 4,423,589, issued to Hardin et al. on Jan. 3, 1984 discloses a method of manufacturing a communication cable by forming a plurality of wire units by advancing groups of twisted wire pairs through twisting stations. Further, U.S. Pat. No. 4,446,689 issued to Hardin et al. on May 8, 1984 relates to an apparatus for manufacturing a communication cable wherein disc frames are provided with aligned apertures in which faceplates movably mounted. During operation, the faceplates are modulated in both frequency and amplitude.
The current materials for use in communications are also well known, for example, U.S. Pat. No. 5,001,304 issued to Hardin et al. on Mar. 19, 1991 relates to a building riser cable having a core which includes twisted pairs of metal conductors. Therein the insulating covers are formed from a group of materials including polyolefin. It should be noted however, that all of the insulating covers are the same and that the flame test used for riser cables is much less severe than the flame test used for plenum cables.
U.S. Pat. No. 5,024,506 issued to Hardin et al. on Jun. 18, 1991 discloses a plenum cable that incudes non-halogenated plastic materials. The insulating material about the metallic conductors is a polyetherimide. Again the insulating material is the same for all of the conductors. Further, in U.S. Pat. No. 5,074,640 issued to Hardin et al. on Dec. 24, 1991 a plenum cable is described that includes an insulator containing a polyetherimide and an additive system including an antioxidant/thermal stabilizer and a metal deactuator. As is the convention, the insulator is the same for all of the metallic conductors.
U.S. Pat. No. 5,202,946 issued to Hardin et al. on Apr. 13, 1993 describes a plenum cable wherein the insulation includes a plastic material. The insulation is the same for all of the conductors within the plenum cable. European Patent 0 380 245 issued to Hardin et al. describes another plenum cable having insulation about the metallic conductors that, in this case, is a plastic material including a polyetherimide. As is the convention the insulation is the same for all of the metallic conductors.
Further, U.S. Pat. No. 4,941,729 describes a cable that is intended as a low hazard cable. This patent describes a cable that includes a non-halogenated plastic material. Similarly, U.S. Pat. No. 4,969,706 describes a cable that includes both halogenated and non-halogenated plastic materials. In both patents the insulating material about the twisted pairs of conductors is the same for each cable.
U.S. Pat. No. 4,412,094 issued to Doughrety et al. on Oct. 25, 1983 relates to a riser cable having a composite insulator having an inner layer of expanded polyethylene and an outer layer of a plasticized polyvinyl chloride. All of the conductors include the same composite insulator.
U.S. Pat. No. 4,500,748 issued to Klein on Feb. 19, 1985 relates to a flame retardant plenum cable wherein the insulation and the jacket are made from the same or different polymers to provide a reduced amount of halogens. This reference tries to predict, mathematically, the performance of cables within the Steiner tunnel. The method does not include fuel contributions or configurations of designs. Further, synergistic effects are not addressed. In each embodiment, the insulation is the same for all of the conductors.
U.S. Pat. No. 4,605,818 issued to Arroyo et al. on Aug. 12, 1986 relates to a flame retardant plenum cable wherein the conductor insulation is a polyvinyl chloride plastic provided with a flame retardant, smoke suppressive sheath system. As is common throughout the known communication cables the conductor insulation is the same for all of the conductors.
U.S. Pat. No. 4,678,294 issued to Angeles on Aug. 18, 1987 relates to a fiber optic plenum cable. The optical fibers are provided with a buffer layer surrounded by a jacket. The cable is also provided with strength members for rigidity.
U.S. Pat. No. 5,010,210 issued to Sidi et al. on Apr. 23, 1991 describes a non-plenum telecommunications cable wherein the insulation surrounding each of the conductors is formed from a flame retardant polyolefin base compound.
U.S. Pat. No. 5,162,609 issued to Adriaenssens et al. on Nov. 10, 1992 relates to a fire-resistant non-plenum cable for high frequency signals. Each metallic member has an insulation system. The insulation system includes an inner layer of a polyolefin and an outer layer of flame retardant polyolefin plastic.
U.S. Pat. No. 5,253,317 issued to Allen et al. on Oct. 12, 1993 describes a non-halogenated plenum cable including twisted pairs of insulated metallic conductors. The insulating material is a non-halogenated polyethersulfone polymer composition. The insulating material is the same for all of the metallic conductors.
It can thus be understood that much time and resources have been dedicated to providing not only communication cables that meet certain safety requirements but adequately meet the electrical requirements as well. Nevertheless, the most common communication cable in use today includes a plurality of twisted pairs of electrical conductors each having an insulation of FEP, which is a very high temperature material and possesses those electrical characteristics, such as, low dielectric constant and dissipation factor, necessary to provide high quality communications cable performance. However, FEP is quite expensive and is frequently in short supply.
Consequently, the provision of a communication cable for use in plenums but has a reduced cost and reduced use of FEP is highly desired.